This invention relates generally to a method of making ultrafine fiber composites using laser assisted reactions. More particularly, this invention relates to a technique for synthesizing ultrafine fiber composites involving the condensation of metallic and nonmetallic species, produced by laser induced evaporation. An example of a composite formed using the method of this invention is an aluminum or tungsten matrix having a dispersed phase of amorphous silica fibers.
In general, composite structures are synthesized to obtain unique mechanical and other properties. This involves appropriate selection of the (1) chemical composition, (2) constituent phases, (3) morphology and (4) structural scale. Achieving the necessary degree of control, particularly in the generation of ultrafine structures, requires using recently developed techniques for the synthesis of thin-layers and small particles. For example, laser methods are frequently employed to produce thin deposited layers by radiation-assisted reactions. Some of the interesting features of the laser reactions and interactions include, (1) high intensity irradiation for rapid heating, (2) selective laser-beam and material coupling, (3) a small interaction zone resulting in localized reactions, thus minimizing contaminations, (4) high rates of chemical and physical transformations of reactants, and (5) generation of mestastable phases.
Presently, three laser-assisted mechanisms have been employed in the surface modification and material synthesis composites. These mechanisms include laser-chemical vapor deposition (LCVD), a laser-liquid reaction, and laser evaporation. The nature of laser evaporation depends upon the coupling of the beam with the material to be evaporated. A material plume is often observed in evaporation. The plume provides active vapor species that can react in a suitable environment and form desirable products which can condense to produce surface layers.
From a practical standpoint of composite synthesis, the control of microstructural scale is critically important. For example, methods leading to a drastic reduction in interfiber spacing will result in increased strength and fracture toughness. Known methods for drastically reducing microstructural scale have produced equiaxed structures as described in the paper presented by R. Birringer, V. Herr and H. Gleiter at the 1986 Fall Material Research Society Meeting (held in Boston, Mass.) wherein the synthesis of 1-100 nm particle materials resulted in unique physical and chemical properties. This prior work involved the synthesis of nanoscale powder materials by evaporation followed by rapid condensation. Conversion into bulk form was then achieved by removal from the processing chamber and powder compaction.